Jitendra one more thing when I put the data in X line item and apply the formula in Y line item then the source of input data would be X line item and I will have to publish the X line item on the dashboard as well then only Y would we updated.
Since you'll be importing data into X line item, and you have Reference X in Y line item. Just Publish Y line item on Dashboard. In this way your user will import the data and will not be able to edit the same.
P.S. : One more thing add this import action into process and publish that process on Dashboard thus whenever the import process in triggered the Dashboard will be refreshed automatically and there will be updated values on Dashboard